Learn Finnish: Ai! Kahvi on kuumaa. - Ouch! The coffee is hot.

In Finnish, the subject of a standard descriptive sentence is usually in the nominative case, even if it is an uncountable mass noun like kahvi. The uncountability is instead shown by putting the adjective describing it into the partitive case, which is why we see kuumaa at the end of the sentence.

Is it ever correct to use the nominative form and say Kahvi on kuuma?

Yes! If you use the nominative adjective kuuma, you are treating the coffee as a single, countable entity rather than a general liquid substance. For example, if you are referring to a specific cup of coffee you just ordered, you could say kahvi on kuuma to mean that specific portion is hot.

Does the exclamation ai always express physical pain?

Not always. While a sharp ai! means ouch and expresses pain, a more relaxed or drawn-out ai is used to express mild surprise or realization, similar to oh or really in English. You will often hear it in everyday conversations in the phrase ai jaa which means oh, I see.